In welding, the mechanical properties of welded joints and of their vicinities are of critical importance. Microstructures and stress distributions in these regions greatly affect the strength of the joints and hence the entire welded structure. Improperly treated welded joints can result in serious structural failure.
For example, in welding of carbon steel, the welded joints and surrounding regions are first heated to produce coalescence and then cooled down. Typically, the cooling rate is sufficiently high so that martensite is formed through a diffusionless phase transformation. Because untempered martensite is typically more brittle than the base steel, and because thermal stress tends to build up during rapid cooling, the welded joints and surrounding regions are prone to fracture if not heat-treated properly.
Traditionally, to improve the strength of welded joints, heat treatment is typically carried out to reduce or eliminate untempered martensite or thermal stress. Heat treatment typically includes an annealing cycle, in which the welded structure is heated and then slowly cooled to the ambient temperature to obtain desired phases and microstructure in the welded joints. For example, electric-resistance welded carbon steel pipes conforming to the standard A53-B § 4.3 set by the American Society for Testing and Materials (“ASTM”) must be treated (commonly referred to as “seam anneal”) after welding to a minimum of about 538° C. (1000° F.) so that no untempered martensite remains or otherwise processed in such a manner that no untempered martensite remains.
The traditional seam anneal is a time- and resource-consuming process. Annealing equipment and power supplies separate from the welding equipment are typically needed. The separate welding and annealing steps contribute to the complexity and length of time of the manufacturing process. In addition, the traditional seam anneal does not consistently remove untempered martensite because the welding seam typically does not follow a perfectly straight line and thus does not always pass the same position in the heating zone in an annealing device, resulting in uneven heat treatment.
The invention disclosed herein is aimed at reducing or eliminating one or more deficiencies in the conventional metal welding technology.